The disclosure of Japanese Patent Application No. 2001-324757 filed on Oct. 23, 2001, including the specification, drawings, and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to an assisting device and method for a variable valve mechanism. More particularly, the invention relates to an assisting device for applying an assisting force acting against a thrust force generated in a control shaft to a variable valve mechanism that allows valve lift amounts to continuously change in such a manner as to interlock with an axial position of the control shaft by axially moving the control shaft.
2. Description of Related Art
As a related art, there is known a variable valve mechanism in which a cam shaft having three-dimensional cams whose cam noses (surfaces) gradually increase in height along an axial direction is moved in the axial direction so as to continuously adjust valve lift amounts of intake valves of an internal combustion engine in accordance with an operational state thereof (Japanese Patent Application Laid-Open No. 2000-54814).
In a variable valve mechanism in which a cam shaft is thus axially moved to allow valve lift amounts to continuously change, a thrust force is generated in such a direction as to reduce the valve lift amounts due to an axial inclination of cam surfaces of three-dimensional cams. Moreover, as the valve lift amounts are increased, compression strokes of valve springs are increased, which leads to a gradual increase in restoring forces thereof. As a result, the aforementioned thrust force is increased as well.
In the case where such a variable valve mechanism is utilized to regulate the amount of intake air in an internal combustion engine by adjusting valve lift amounts of intake valves instead of adjusting a throttle valve, an actuator for axially moving a cam shaft is required to have high responding properties. Especially in the case where a hydraulic actuator is employed, in order to accomplish high responding properties, it is required that the flow rate of a hydraulic fluid be reduced by reducing the diameter of pistons. However, if the diameter of the pistons is reduced, the actuator output cannot be adapted for an increase in the aforementioned thrust force, which causes an apprehension that a minimum hydraulic fluid pressure will not be generated or that the responding properties will deteriorate.
In order to address these problems, one might consider providing an assisting spring for assisting the operation of the actuator by generating an assisting force that acts against the aforementioned thrust force. However, as described above, while the thrust force is increased in proportion to an increase in the valve lift amounts, the restoring force of the assisting spring is reduced as the cam shaft is shifted to the high-lift side. Hence, this restoring force is inadequate as an assisting force.
Such a problem is caused in other types of variable valve mechanisms in which valve lift amounts can continuously change due to axial movements of a control shaft, as well as in a variable valve mechanism employing three-dimensional cams.
It is an object of the invention to provide an assisting device capable of applying a suitable assisting force to a variable valve mechanism that allows valve lift amounts to continuously change with changes in an axial position of a control shaft by axially moving the control shaft.
In order to achieve the aforementioned and/or other objects, an assisting device for applying an assisting force to counteract a thrust force generated in a variable valve mechanism according to one aspect of the invention comprises valves disposed in the variable valve mechanism, a control shaft for allowing valve lift amounts of the valves to continuously change with changes in an axial position of the control shaft, the control shaft receiving the thrust force from the valves, and an assisting force applying portion for generating and applying the assisting force on the basis of a restoring force of an elastic body or a pressure of a fluid and increasing the assisting force as the axial position of the control shaft is shifted to a high-lift side.
This structure allows a suitable assisting force capable of acting against a thrust force that is increased as the axial position of the control shaft is shifted to the high-lift side to be applied to the variable valve mechanism.